One-piece monolithic cuff and artificial urinary sphincter system

ABSTRACT

An artificial urinary sphincter system has a pressure reservoir attachable to a pump and a conduit attachable between the pump and a cuff. The cuff is formed as a one-piece monolithic polymer structure that is substantially circular in radial cross-section. The cuff has an interior wall connected by a pair of opposing end walls to an exterior wall and includes a closed cavity located between the interior wall and the exterior wall. The cuff has a part line that allows for placement of the cuff around a urethra of a user.

BACKGROUND

Urinary incontinence affects about 200 million people worldwide andabout 25 million people in the US. Urinary incontinence is generallymore prevalent in women than in men.

Urinary incontinence in women can be associated with a prolapse of oneor more pelvic organs, which can arise from a weakness in thetissues/muscle of the pelvic floor. Urinary incontinence in men canarise after surgical treatment of the prostate glade, which treatmentcan include removal or weakening of the prostatic sphincter associatedwith the urinary urethra.

One treatment for urinary incontinence includes placing an artificialsphincter around a portion of the urethra. The artificial sphincter hasa closed position that selectively prevents the flow of urine throughthe urethra, thus providing the user with a comfortable, continentstate. The artificial sphincter can be activated to an open position bythe user, which opens the urethra and allows the user to selectivelypass urine.

Surgeons and patients would welcome advances in the treatment of urinaryincontinence.

SUMMARY

One aspect provides an artificial urinary sphincter system having apressure reservoir attachable to a pump and a conduit attachable betweenthe pump and a cuff. The cuff is formed as a one-piece monolithicpolymer structure that is substantially circular in radialcross-section. The cuff has an interior wall connected by a pair ofopposing end walls to an exterior wall. The cuff includes a closedcavity located between the interior wall and the exterior wall andbetween the pair of opposing end walls. The one-piece monolithic polymerstructure has a part line that is formed by a first face of the cuffthat is separable from a second face of the cuff to allow the first faceto be separated from the second face for placement of the cuff around aurethra of a user. The conduit is attachable to a connector that isattached to the exterior wall of the cuff adjacent to one of the firstface or the second face. The connector communicates with the closedcavity. A thickness of the interior wall is less than a thickness of theexterior wall and less than a thickness of each of the pair of opposingend walls.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1A is a side view of a prior art cuff and FIG. 1B is a perspectiveview of an artificial urinary sphincter (AUS) system with the prior artcuff.

FIG. 2 is a perspective view of one embodiment of an AUS systemimplanted in the environment of the male urogenital region.

FIG. 3 is a perspective view of one embodiment of a cuff of the AUSsystem illustrated in FIG. 2.

FIG. 4 is in axial cross-sectional view of the cuff illustrated in FIG.3.

FIG. 5 is a radial cross-sectional view of the cuff illustrated in FIG.3.

FIG. 6 is a perspective view of one embodiment of a cuff suitable foruse with the AUS system illustrated in FIG. 2.

FIG. 7 is a perspective view of one embodiment of a cuff suitable foruse with the AUS system illustrated in FIG. 2.

FIG. 8 is a perspective view of a surgical instrument inserted into andexpanding the cuff illustrated in FIG. 2.

FIG. 9 is a schematic view of the cuff illustrated in FIG. 2 placedaround the urethra of a patient.

FIG. 10 is a schematic view of the cuff illustrated in FIG. 9 afterimplantation.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

Tissue includes soft tissue, which includes dermal tissue, sub-dermaltissue, ligaments, tendons, or membranes. As employed in thisspecification, the term “tissue” does not include bone.

As employed in this specification, the term “end” means endmost or thevery end point of the subject being described, and the term “endportion” means that segment that is immediately adjacent to the end ofthe subject being described.

FIG. 1A is a side view of a prior art cuff 20 and FIG. 1B is aperspective view of an artificial urinary sphincter (AUS) system 30including the prior art cuff 20. The cuff 20 includes a backing 22, aninflatable balloon 24 attached to the backing 22, and a conduit thatcommunicates with the inflatable balloon 24. The backing 22 is generallyinextensible and configured to retain the cuff 20 in position as theinflatable balloon 24 is inflated to coapt the urethra.

The system 30 includes a pressure-regulating reservoir 32 attached to apump 34 by tubing 36 with the cuff 20 attached to the pump 34 byseparate tubing 38. The pump 34 is implantable in the scrotum of a maleuser. The pressure-regulating reservoir 32 is implantable in theprevesical space posterior the rectus fascia and the linea alba. Thesystem 30 is filled with liquid (e.g., saline) to provide aliquid-filled closed system maintained at an equilibrium pressure thatis selected to close the cuff 20 around the urethra. When the userwishes to void, he squeezes and releases the pump 34 several times tomove the liquid from the cuff 20 into the pressure-regulating reservoir32. The cuff 20 “deflates” and opens, which allows the urethra to openand pass urine. The pressure-regulating reservoir 32, having beenpressurized to a pressure above the equilibrium pressure by action ofthe pump 34, eventually returns to the equilibrium pressure over thecourse of several minutes to again inflate the cuff 20 and coapt theurethra.

With reference to FIG. 1B, the cuff 20 is illustrated with one endwrapped around to meet the second end to allow the cuff 20 to surroundthe urethra. When the generally rectangular cuff 20 (FIG. 1A) is formedaround the urethra, the inflatable balloons 24 become bunched togetherand unavoidably become creased. The bunching together of the inflatableballoons 24 can lead to the development of regions of stress 40 in theballoon-forming layer. The regions of stress 40 can potentially causethe balloon-forming layer to fail, resulting in a leakage in the system20. The bunching together of the inflatable balloons 24 also has thepotential to undesirably pinch or erode the sensitive tissue of theurethra.

Embodiments described in this application provide a cuff that is formedas a one-piece monolithic structure that is substantially circular inradial cross-section. The one-piece monolithic structure has a smoothinterior wall providing an absence of creases and an absence ofconcentrated stress regions on an interior surface of the cuff. Theone-piece monolithic structure includes a closed cavity that is filledwith liquid to inflate and deflate the cuff. Embodiments provide aninterior wall having a thickness that is less than a thickness of theexterior wall of the monolithic cuff. In this manner, the exterior wallsupports the cuff (or provides a supporting backbone) as the cuff isinflated, which allows the interior wall to expand to coapt the urethra.The smooth interior wall inflates evenly and uniformly to reduce theformation of stress points that can undesirably lead to leakage of thesaline through the cuff, and ensures improved coaptation of the urethrawith reduced or no pinching of the urethral tissue.

FIG. 2 is a perspective view of one embodiment of an artificial urinarysphincter (AUS) system 100 illustrated as implanted in the environmentof the male urogenital region. The AUS system 100 includes a pump 102attachable with tubing 104 between a pressure regulating reservoir 106and a cuff 108.

The pump 102 is typically implanted within the scrotum S, which providesconvenient access to the pump 102 by a male user. Other locations forplacement of the pump 102 are also acceptable, for example within thelabia as determined by the gender of the user.

The pump 102 is operable to inflate the cuff 108. One suitable pump 102is the suction style pump bulb as illustrated that operates to removeliquid from the cuff 108 and transport the liquid into the reservoir106. Other suitable pump mechanisms are also acceptable, includingelectromechanical pumps, electronic pumps, button-style displacementpumps, and the like.

The tubing 104 is provided in a kink resistant form and includes somestyle of connector that allows segments of the tubing 104 to be attachedto the system components (the reservoir 106 and the cuff 108) after thesystem 100 is primed with liquid. The tubing 104 is generally athin-walled tube that is attachable between the pump 102 and thereservoir 106, and between the pump 102 and the cuff 108. In oneembodiment, the tubing 104 is separate from the pump 102 and separatefrom the reservoir 106/cuff 108 and connects to these components througha locking mechanism, such as a quick connector or other suitablesnap-fit connector.

The reservoir 106 is sized to retain a volume of liquid at anequilibrium pressure that is selected to inflate the cuff 108 forcoaptation of the urethra. The reservoir 106 maintains the pressure inthe closed system 100 after the tubing 104 is connected between thecomponents.

The cuff 108 is implanted around the bulbous urethra or around theportion of the urethra descending from the bladder neck N. The cuff 108is sized to allow placement as close to the bladder B as possible(desired by some surgeons), or positioned distal the bladder neck N assuitably determined by the attending surgeon. As illustrated in FIG. 2,the cuff 108 is implanted around the urethra U at a location where theurethra U transitions from a vertical orientation communicating with thebladder B to a horizontal orientation extending to the penis P, whichdesirably corresponds to the area of the urogenital region associatedwith an increased level of muscle M mass.

FIG. 3 is a perspective view of one embodiment of the cuff 108. The cuff108 is formed as a one-piece monolithic polymer structure that issubstantially circular in radial cross-section (see FIG. 5).

The cuff 108 includes an interior wall 110 connected by a pair ofopposing end walls 114, 116 to an exterior wall 112.

The cuff 108 includes a part line 120 formed longitudinally in an axialdirection between the ends 114, 116. The part line 120 is formed by afirst face 122 of the cuff 108 that is adjacent to or in contact with asecond face 124 of the cuff 108. The part line 120 allows the cuff 108to be “opened” for positioning around the urethra by separating thefirst face 122 from the second face 124. When the cuff 108 is implanted(or in its rest state), the first face 122 touches or is very close tothe second face 124 such that the cuff 108 maintains its circularcross-sectional shape.

The cuff 108 includes a connector 130 that is in fluid communicationwith a cavity formed between the interior wall 110 and the exterior wall112. The connector 130 is attached to the tubing 104 that extends to thepump 102 (FIG. 2). When the cuff 108 is implanted into a patient, thepart line 120 is opened to secure the cuff 108 around the urethra. It isdesirable for the surgeon to have access to the connector 130 after thecuff 108 is implanted, and for this reason the connector 130 is attachedto the exterior wall 112 adjacent to the second face 124 of the partline 120. After implantation, the part line 120 will be orientedinferior to the urethra (standing position, or anterior the urethra)between the urethra and the perineum; positioning the connector 130 nearthe part line 120 provides the surgeon with optimal access to theconnector 130.

In one embodiment, the lengths of the walls 110, 112 are about equal tothe diameter of the cuff 108 such that the cuff 108 is “donut” shaped orin the shape of a torus. In one embodiment, the lengths of the walls110, 112 are substantially greater than the diameter of the cuff 108such that the cuff 108 is cylindrical in shape.

The cuff 108 is fabricated as a one-piece monolithic structure and is soconfigured that the interior wall 110 is smooth and wrinkle free in boththe axial direction and in the radial direction around the cuff 108.Suitable fabrication processes for forming the cuff 108 include polymermolding, injection molding, lost wax molding, machining,three-dimensional printing, and other forms of forming an integratedone-piece structure. Suitable material for fabricating the cuff 108includes polymers suitable for use in the fabrication process above,including silicone, polyurethane, polybutylene, and blends and mixturesof some or all of these materials.

FIG. 4 is an axial cross-sectional view and FIG. 5 is a radialcross-sectional view of the cuff 108. The cuff 108 is an inflatable cuffand includes a closed cavity 140 formed between the interior wall 110,the exterior wall 112, and the end walls 114, 116. The closed cavity 140communicates with the pump 102 and the reservoir 106 (FIG. 2) throughthe connector 130. The interior wall 110 is configured to expand inwardrelative to the cuff 108 when the liquid in the closed cavity 140 ispressurized by the pressure within the pressure regulating reservoir 106(FIG. 2). In one embodiment, the interior wall 110 has a thickness Tithat is less than the thickness Te of the exterior wall 112. In oneembodiment, the interior wall 110 has a thickness Ti that is less thanthe thickness Te of the exterior wall 112 and less than a thickness ofthe end walls 114, 116. In one embodiment, the thickness Ti of theinterior wall 110 is less than one half of the thickness Te of theexterior wall.

FIG. 4 illustrates an embodiment having curved end walls 114, 116. Inone embodiment, the end walls 114, 116 are formed by a constant radius rcurvature. The axial cross-sectional view illustrated in FIG. 4illustrates that the cuff 108 is cylindrical and non-circular in itsaxial direction, which provides a length that coapts a greater length ofurethra compared to a donut shaped cuff. Both the interior wall 110 andthe exterior wall 112 are substantially smooth in the axial directionand characterized by an absence of creases and an absence of folds. Itis desirable that the cuff 108 is fabricated in a shape that representsthe form that will be used when the cuff 108 is implanted. Embodimentsprovide a cuff that is pre-stressed when fabricated to maintain asubstantially circular radial cross-sectional shape having a smoothinterior wall.

FIG. 6 is a perspective view of one embodiment of a cuff 208. The cuff208 is similar to the cuff 108 described above and is fabricated as aone-piece monolithic polymer structure with a part line 220 and theconnector 130 communicating with the internal closed cavity (not shown).The cuff 208 includes a closure mechanism provided by a first suturehole 230 that is formed in the end of the cuff 208 and extending throughthe first face 222 and a second suture hole 232 that is formed in theend of the cuff 208 and extending through the second face 224. Thesuture holes 230, 232 are provided to allow the surgeon to place asuture through the holes 230, 232 to retract and hold the opposing faces222, 224 together. In one embodiment, a first set of suture holes 230,232 is provided on a first end of the cuff 208 and a second set ofsuture holes 240, 242 is provided on the opposing end of the cuff 208.

FIG. 7 is a perspective view of one embodiment of a cuff 308. The cuff308 is similar to the cuff 108 described above and is fabricated as aone-piece monolithic polymer structure with a part line 320 and theconnector 130 communicating with the closed cavity (not shown). The cuff308 is fabricated to include a closure mechanism provided by a tongue330 attached to the exterior wall 312 adjacent to, for example, thefirst face 322 of the part line 320 and a keeper 332 attached to theexterior wall 312 adjacent to the second face 324 of the part line 320.The tongue 330 is insertable through the keeper 332 to secure the cuff308 around the urethra. In one embodiment, the tongue 330 includes a setof teeth that engage with a ratchet formed on the inside of the keeper332. Other forms of retaining the tongue 330 inserted into the keeper332 are also acceptable.

FIG. 8 is a perspective view of a surgical tool 400 employed to open thecuff 108 for placement around a urethra. The surgical tool 400 caninclude a forceps, a scissors, a blunt tip scissors, or the like that isinserted into the opening of the cuff 108 to apply pressure to theinterior wall 110 to force the first face 122 away from the second face124 to expand the part line 120. It is desirable to open the part line120 a sufficient distance to allow the cuff 108 to be placed over theoutside diameter of the urethra. The separable part line provides meansfor opening the circular shape of the cuff to allow placing the cuffaround the urethra of the user. The cuff is fabricated to bepre-stressed or configured to have a memory that returns the cuff 108 toits circular (or closed) shape. Therefore, when the surgical tool 400 isremoved the part line 120 closes.

FIG. 9 is a schematic view of a perineal incision PI formed to exposethe urethra U, with the cuff 108 placed around the urethra U. The partline 120 has been expanded as described above and the cuff 108 has beenpositioned around the urethra U such that the connector 130 isaccessible by the surgeon. In anatomical terms, the part line 120 andthe connector 130 are located inferior relative to the urethra U betweenthe perineum P and the urethra U.

With reference to FIG. 2 and FIG. 9 as descriptive examples, a method oftreating a patient suffering from urinary incontinence includes asurgical procedure to implant the system 100. The patient is preparedfor surgery in a manner described by the hospital or clinic policies oras supervised and approved by the surgeon. The perineal area of thepatient is cleaned with suitable cleansers and prepared for surgery. Aperineal incision PI is made on the midline of the patient, and tissueis dissected to expose the bulbospongiosus muscle supporting the urethraU. The surgeon will dissect laterally to free the fascia around thebulbospongiosus muscle and expose a portion of the bulbar urethra U. Thebulbospongiosus muscle is immobilized, for example by clamping laterallyto each side of the patient (FIG. 9), which exposes the urethra U foraccess by the surgeon. The surgeon dissects additional tissue and muscleby “tunneling” around the posterior side of the urethra U to create apathway around a circumference of the urethra. The cuff 108 is opened ina manner as illustrated in FIG. 8 by separating the first face 122 fromthe second face 124 to expand the part line 120 enough to allow theurethra U to be surrounded by the cuff 108. The cuff 108 is placedaround the urethra U with the connector 130 of the cuff 108 anterior tothe urethra U. The cuff 108 is secured around the urethra U by sutures(FIG. 7) or a strap (FIG. 8). The connector 130 is attached to thetubing 104.

The surgeon typically confirms performance of the cuff 108 by injectingliquid into the connector 130 to fill the closed system 100, usuallyimmediately prior to implantation of the system 100. The tubes 104 andthe pump 102 are subsequently attached to the cuff 108. For example, thepump 102 is located in the scrotum (male) or labia (female) of the userand the tube 104 is connected between the pump 102 and the cuff 108placed around the urethra U. The surgeon will pressurize the system 100to the equilibrium pressure and cause the cuff 108 to coapt the urethraU. The surgeon will confirm that operation of the pump 102 will moveliquid from the cuff 108 to the pressure-regulating reservoir 106, thusopening the urethra U and allowing the patient to pass urine. Thesurgical site is closed after confirmation of performance of the system100.

FIG. 10 is a schematic view of the cuff 108 after implantation andclosing of the perineal incision PI. The part line 120 formed throughthe walls 110, 112 of the cuff 108 is located anterior relative to theurethra U such that the connector 130 is easily accessed by the surgeonduring the surgical procedure. The system 100 illustrated in FIG. 2 isselectively operated by the user to inflate the cuff 108 to close theurethra (as shown in FIG. 10) or to deflate the cuff 108 to open theurethra to allow the passing or urine. In one embodiment described withreference to FIG. 2, the pressure-regulating reservoir 106 is filled andpressurized by the surgeon to a suitable pressure P upon implantationsuch that the system 100 maintains the urethra U in the closed positionwhen the system 100 is in the steady state. The pressure-regulatingreservoir 106 pressurizes the cuff 108 to close the urethra U andprovide urinary continence for the user. The pressure P in the cuff 108forces the wall 110 inward against the urethra U. The exterior wall 112is thicker and stiffer and more resistant to deformation that thethinner interior wall 110, and as such, the wall 112 provides abackboard for supporting the movement of the interior wall 110. When theuser desires to pass urine, the user squeezes the pump bulb 102 to movethe liquid from the closed cavity 140 to the reservoir 106, whichdeflates the interior wall 110 of the cuff 108 and allows the passage ofurine.

The wall 100 is configured to uniformly press against the urethra tosmoothly coapt the urethra. FIG. 10 illustrates that pinching of theurethral tissue is reduced or eliminated with this design. The smoothand uniform nature of the wall 110, 112 configurations also reduces thelikelihood of the cuff 108 forming a leak that can arise when inflatableAUS walls are bunched together, and this improved design reduces thepossibility of cuff leakage from the cyclic use of the cuff 108 overmany months.

Embodiments also include an alternative steady state in which theurethra U is open when the cuff 108 is in the steady state, andoperation of the pump 102 operates to pressurize the cuff 108 and closethe urethra.

Although specific embodiments have been illustrated and described inthis patent application, it will be appreciated by those of ordinaryskill in the art that a variety of alternate and/or equivalentimplementations may be substituted for the specific embodiments shownand described without departing from the scope of the invention. Thispatent application is intended to cover any adaptations or variations ofmedical devices, as discussed above. Therefore, it is intended that thisinvention be limited only by the claims and their equivalents.

What is claimed is:
 1. An artificial urinary sphincter systemcomprising: a pressure reservoir attachable to a pump and a conduitattachable between the pump and a cuff, the cuff formed as a one-piecemonolithic polymer structure that is substantially circular in radialcross-section, the cuff having an interior wall connected by a pair ofopposing end walls to an exterior wall, the cuff including a closedcavity located between the interior wall and the exterior wall andbetween the pair of opposing end walls; wherein the one-piece monolithicpolymer structure has a part line that is formed by a first face of thecuff that is separable from a second face of the cuff to allow the firstface to be separated from the second face for placement of the cuffaround a urethra of a user; and wherein the conduit is attachable to aconnector that is attached to the exterior wall of the cuff adjacent toone of the first face or the second face, the connector communicatingwith the closed cavity.
 2. The system of claim 1, wherein a thickness ofthe interior wall is less than a thickness of the exterior wall and lessthan a thickness of each of the pair of opposing end walls.
 3. Thesystem of claim 1, wherein the cuff is cylindrical in axialcross-section, and the interior wall and the exterior wall are smoothand connected by a pair of opposing curved end walls.
 4. The system ofclaim 1, wherein each of the opposing end walls is curved.
 5. The systemof claim 4, wherein each of the curved end walls is formed by a constantradius curve.
 6. The system of claim 1, wherein the cuff furthercomprises a first suture hole formed in the first face and a secondsuture hole formed in the second face and so configured to allow thefirst face of the cuff to be secured to the second face of the cuff. 7.The system of claim 1, wherein the cuff further comprises a tongueattached to the exterior wall adjacent to one of the first face and thesecond face and a keeper attached to the exterior wall adjacent to another of the first face and the second face, the tongue insertable intothe keeper to secure the cuff around the urethra of the user.
 8. Thesystem of claim 1, wherein the interior wall of the cuff is smooth in aradial direction and characterized by an absence of creases.
 9. Thesystem of claim 1, wherein the interior wall of the cuff is smooth in anaxial direction and characterized by an absence of folds.
 10. The systemof claim 1, wherein the cuff is formed as a one-piece monolithicsilicone structure.
 11. The system of claim 1, wherein the cuff ispre-stressed to be substantially circular in the radial cross-section.12. The system of claim 1, wherein the cuff is molded to maintain thesubstantially circular radial cross-section.
 13. The system of claim 1,wherein the thickness of the interior wall is less than half of thethickness of the exterior wall.
 14. The system of claim 1, wherein theinterior wall and the exterior wall are smooth and continuous in both aradial direction and an axial direction.
 15. An artificial urinarysphincter system comprising: a pressure reservoir attachable to a pumpand a conduit attachable between the pump and a cuff, the cuff formed asa one-piece monolithic structure that is substantially circular inradial cross-section, the cuff having an interior wall connected by apair of opposing end walls to an exterior wall, the cuff including aclosed cavity located between the interior wall and the exterior walland between the pair of opposing end walls; wherein the cuff iscylindrical with an axial direction that is longer than a radialdirection of the cuff, and the interior wall forms a circle in radialcross-section that is smooth and configured to expand uniformly inwardrelative to the cuff; and wherein the one-piece monolithic structure hasa part line that is formed by a first face of the cuff that is separablefrom a second face of the cuff to allow the first face to be separatedfrom the second face for placement of the cuff around a urethra of auser.
 16. The system of claim 15, wherein the conduit is attachable to aconnector that is attached cuff adjacent to one of the first face or thesecond face, the connector communicating with the closed cavity.
 17. Anartificial urinary sphincter system including a pressure reservoirattachable to a pump and a conduit attachable between the pump and acuff, the cuff comprising: a one-piece monolithic silicone structurethat is pre-formed to provide a circular shape in radial cross-sectionthat is sized for placement around a urethra of a user; an interior wallconnected by a pair of opposing curved end walls to an exterior wall,the interior wall and the exterior wall both smooth and continuous inboth a radial direction and an axial direction; a closed cavity locatedbetween the interior wall and the exterior wall and between the pair ofopposing curved end walls; means for opening the circular shape of thecuff to allow placing the cuff around the urethra of the user; and aconnector that is attached to the exterior wall of the cuff and incommunication with the closed cavity; wherein a thickness of theinterior wall is less than a thickness of the exterior wall and lessthan a thickness of each of the pair of opposing curved end walls.